Intoxicant consumed by Gareth, an Olympian (7)
I believe the answer is:
ethanol
'intoxicant' is the definition.
(I've seen this before)
'consumed by gareth an olympian' is the wordplay.
'consumed by' indicates the answer is hidden within the clue (I've seen this in other clues).
'ETHANOL' is hidden in the letters of 'gareth an olympian'.
(Other definitions for ethanol that I've seen before include "intoxicating stuff" , "Intoxicant in wine, spirits, etc" , "volatile flammable substance" , "porter uses it" , "Alcohol in wine, beer" .)